Blood Depots

reservoir-type organs that in higher animals and humans can store, aside from the general bloodstream, about 50 percent of all the blood in the body. When the body’s demand for oxygen increases (for example, during heavy physical exertion) or the amount of hemoglobin in circulating blood decreases (for example, as a result of blood loss), blood from the blood depots enters into general circulation. The main depots are the spleen, liver, and skin. The ability of these organs to function as blood depots is due to the peculiar structure of their vascular systems. In the spleen, some of the blood penetrates into the intercellular spaces, where it is isolated from the general circulation. Blood flows back into the bloodstream when the smooth muscles of the spleen contract. In the liver, blood is retained because more flows into it than out of it. The liver is freed of excess blood by acute constriction of the vessels that transport blood to it. In the skin, blood is stored in the subpapillary plexes of the capillaries (parallel branches from the main blood vessel of the skin), where the blood flows continuously.